Michael T. Duignan

Beam cleanup with stimulated Raman scattering in the intensity-averaging regime

We describe experimental and theoretical investigations of beam cleanup with highly aberrated pump beams in the intensity-averaging regime. Distortion-free amplification of a diffraction-limited Stokes beam is demonstrated in a crossed-beam geometry with a pump beam aberrated to 120 times its diffraction limit, resulting in a brightness increase of 5000 times. Moderately aberrated pump beams produce off-axis Stokes components, while collinear interactions introduce distortion on the Stokes beam. Phase conjugation is combined with stimulated Raman scattering to remove both the aberrations of the pump beam and the aberrations on the Stokes beam itself.

Stimulated Brillouin scattering and loop threshold reduction with a 2.1-μm Cr,Tm,Ho:YAG laser

We have investigated stimulated Brillouin scattering (SBS) in carbon disulfide, using a frequency-narrowed Cr,Tm,Ho:YAG laser operating at 2.12 μm. Threshold reduction with a novel loop geometry is also demonstrated. A model for the loop-SBS scheme is developed and compared with experimental results. Backscatter power, energy, and conjugate-fidelity data are presented.

Laser-based sample preparation for electronic package failure analysis

Failure analysis has come to play a key role in ensuring quality and reliability in semiconductor devices, associated packaging and printed wiring boards. Tools are increasingly available to those investigating high-density integrated circuits at the die level, particularly for edit and repair operations. Until recently however, this capability has been limited by the inherent low-resolution mechanical/manual processes used for destructive analysis on electronics packaging. A laser-based tool has been developed to selectively and locally enable access to traces and layers within packages and provide a way to perform edits to an area of interest.

An Integrated Tool for Rapid Prototyping of Electronic Circuits Using a Laser Direct Write Technique

A laser-based tool has been developed for the fabrication of electronic circuits. The tool integrates three separate functions on a single platform. These functions are deposition, laser processing, and laser micromachining. Deposition is accomplished by dispersing a target material in a thin layer over a transparent backing layer, holding the target material in close proximity to a receiving substrate, and irradiating the target material from behind with a short pulse UV laser. The UV pulse vaporizes a small amount of the material at the material/backing interface, thereby propelling the remainder of the target material toward the receiving substrate. Patterning is achieved either by translating the receiving substrate, scanning the laser beam, or a combination of the two. After transfer, most materials require some thermal processing: either oven baking or laser sintering. Fabricated circuits have included conductors (Ag, AgPd, AgPt, Cu), resistors (cermet and polymer thick film), and dielectrics (ceramic and polymer thick film). These materials have been patterned with feature sizes as small as 10 m and linear write speeds as high as 1 meter per second. The tool includes an integrated, near IR laser which can be operated both CW and pulsed for thermal processing. Laser sintering of metals, cermet resistors, and ceramic dielectrics has been demonstrated on low temperature, polymer substrates. The localized heating achievable with the laser allows the sintering or annealing of high temperature materials without damaging the underlying substrate. The tool includes a complete, state-of-the-art laser micromachining system, capable of milling recesses, drilling vias, trimming components, and excising circuits. No masks or phototools are required. No photoresists or wet chemistries are needed. The tool does not require a clean room environment. The combined functionality and speed result in a flexible and powerful tool for the rapid prototyping of circuits, and potentially, small scale production.

Applications Of Raman Beam Cleanup And Phase Conjugation To The Reduction Of Laser Beam Aberrations

We describe the use of Raman beam cleanup for reducing the divergence of laser beams. Distortion-free amplification of a diffraction-limited Stokes beam with a pump beam aberrated to 120 times its diffraction limit is demonstrated. With a power conversion efficiency of 30% the resulting increase in brightness was 5000 times. We also demonstrate the combination of phase conjugation and Raman beam cleanup to remove distortions on the Stokes beam. Results are compared with theory.

Use Of Stimulated Raman Scattering For Reducing The Divergence Of Severely Aberrated Laser Beams

Amplification of a diffraction-limited Stokes beam in a hydrogen Raman amplifier pumped by a severely aberrated XeC1 laser has been studied experimentally. Spatial quality of the amplified Stokes beam and conversion efficiency from the pump laser were measured. Numerical study of this process using a two-dimensional propagation code that includes pump depletion was performed. The simulation shows that the quality of the amplified Stokes beam depends critically on the absence of any near-axial components in the pump laser. These components of the pump can cause phase matched four-wave mixing interactions with the Stokes, leading to increased angular divergence of the amplified Stokes beam and the development of secondary sidebands in the far field. An optical integrator was used to focus the poor quality pump beam into the amplifier and to remove all near-axial components in the pump field. The diffraction-limited nature of the input Stokes beam was then preserved in the amplified Stokes beam. A power conversion from the aberrated pump to the Stokes beam of the order of 30% was observed. The far field intensity of the Stokes beam increased by a factor of 5000 over that of the original pump.

Phase conjugation of 2.91 μm HF laser radiation via stimulated Brillouin scattering

A flashlamp‐pumped pulsed hydrogen fluoride laser, operating single‐line at 2.91 μm, was used to generate backward stimulated Brillouin scattering in high pressure xenon gas with power reflectivities of ∼50%. Far field spatial profiles indicate correction of random phase front aberrations through phase conjugation via the SBS process.

High reflectivity SBS of 2.9 μ HF laser radiation in xenon

We have successfully demonstrated, for the first time, stimulated Brillouin scattering of single‐line HF laser radiation in high pressure xenon. The temporal behavior of the input, transmitted and SBS reflected laser pulses was monitored. Energy and power reflectivities were also measured as a function of input laser power, xenon pressure, and f‐number of focusing optics. Brillouin reflection could be unambiguously distinguished from much weaker reflections due to occasional optical breakdown. Power reflectivities of greater than 50% have been observed.

Pump Replication and Related Effects in Raman Beam Cleanup

We examine conditions under which pump beam replication can occur in Raman beam cleanup with crossed pump beams. The dependence of pump beam replication on the small signal Raman gain and on the Stokes seed energy is explored.

Nonlinear Optical Phase Conjugation of HF Laser Radiation Via Stimulated Brillouin Scattering

Stimulated Brillouin scattering of hydrogen fluoride laser radiation has been demonstrated for the first time. A flashlamp-pumped pulsed chemical laser, operating single-line at 2.91 μm, was used to generate backward stimulated Brillouin scattering in high-pressure xenon gas with power reflectivities of ~50%. Intentional aberration and subsequent restoration of high spatial quality beam provided evidence of phase conjugation by the SBS process.